The present invention generally relates to a positive electrode active material for a lithium ion battery, and particularly relates to a positive electrode active material, a positive electrode for a secondary battery, and a lithium ion battery having high crystallinity and high safety while being able to ensure high capacity.
In recent years, there is a rapidly growing demand for a non-aqueous electrolyte lithium secondary battery as a high energy density battery. This lithium secondary battery is configured from three fundamental components; namely, a positive electrode, a negative electrode, and a separator retaining an electrolyte interposed between these electrodes.
As the positive electrode and negative electrode, a slurry obtained by mixing and dispersing active materials, conductive materials, bonding materials and plasticizing agents (where appropriate) in a dispersion medium is used by being supported by a collector such as a metallic foil or a metallic mesh.
Under the foregoing circumstances, as the positive electrode active material, composite oxide of lithium and transition metal; in particular, cobalt-based composite oxide, nickel-based composite oxide, and manganese-based composite oxide are typical examples. These lithium composite oxides are generally synthesized by mixing the compound of the constitution element (carbonate and oxide of Mn, Fe, Co, Ni and the like) and lithium compound (lithium carbonate and the like) at a predetermined ratio, and performing heat treatment (oxidation treatment) thereto (refer to Patent Document 1, Patent Document 2, and Patent Document 3).
Under the foregoing circumstances, a ternary positive electrode material of Ni:Mn:Co=1:1:1 composition has been proposed (refer to Patent Document 4). In the case of Patent Document 4, the Li/metal ratio is 0.97 to 1.03, and it describes that it is possible to obtain a discharge capacity of 200 mAh/g. Nevertheless, in the foregoing case, since the charge cutoff voltage is a high voltage of 4.7 V, if the voltage is cut at 4.3 V, the initial discharge capacity will be roughly 150 mAh/g.
Generally speaking, the initial performance, cycle life or internal resistance of a battery will differ considerably depending on the crystal structure of the material. Even if the material is of a layered structure, there is a problem in that the battery performance will deteriorate if a spinel structure or the like coexists locally.
Thus, the identification of the crystal structure is important, but the identification of the crystal structure was conventionally based on XRD (X-ray diffraction). Nevertheless, it was difficult to determine the coexistence of phases due to reasons such as the peak positions being close.
In light of the above, a proposal has been made for defining the positive electrode active material based on Raman spectrometry (refer to Patent Document 5). Patent Document 5 defines the peak intensity ratio of the spinel structure and the hexagonal structure in the Raman spectrum analysis based on the chemical formula of LiCoMA2 (0.95≦Li≦1.0, wherein A contains O, F, S, P). However, since the main peak is the peak of the spinel structure and not a layered structure, it cannot be said that sufficient performance have been obtained.
As described above, a lithium secondary battery material yields superior performance compared to conventional technology, but further improvement is demanded in terms of sinterability and battery performance.
Layered lithium nickel-manganese-cobalt composite oxide as a positive electrode active material for a lithium ion battery has high expectations, since it has high capacity and high safety compared to lithium cobalt oxide and lithium manganese oxide. Nevertheless, there is limited literature regarding its composition and crystallinity, and with respect to the lattice constant in particular, descriptions are provided only in terms of its approximate width.
There are the following Patent Documents (refer to Patent Documents 6 to 9) that define the lattice constant in a positive electrode active material for a lithium ion battery having a composition of LiaNixMnyCozO2.
For example, Patent Document 6 describes a lithium-nickel-manganese cobalt-based composite oxide for a lithium secondary battery positive electrode material in which the lattice constant is within the range of 2.855 Å≦a≦2.870 Å, 14.235 Å≦c≦14.265 Å. Patent Document 7 describes a layered lithium nickel-based composite oxide in which the bulk density is 2.0 g/cc or more, the median size of secondary particles is 9 to 20 um, and the BET specific surface area is 0.5 to 1 m2/g. Patent Document 8 describes a lithium-containing transition metal composite oxide in which the lattice constant of the a axis is 2.895 to 2.925 Å, and the lattice constant of the c axis is 14.28 to 14.38 Å. Patent Document 9 describes a lithium-containing transition metal composite oxide in which the lattice constant of the a axis is 2.830 to 2.890 Å, and the lattice constant of the c axis is 14.150 to 14.290 Å.
Nevertheless, although the lattice constant of the a axis and the lattice constant of the c axis are defined, there is no in-depth description regarding the composition and molar volume. Thus, there is a problem in that the foregoing Patent Documents are insufficient in terms of ensuring the performance and safety of the lithium ion battery.    Patent Document 1: Japanese Published Unexamined Patent Application No. H1-294364    Patent Document 2: Japanese Published Unexamined Patent Application No. H11-307094    Patent Document 3: Japanese Published Unexamined Patent Application No. 2005-285572    Patent Document 4: Japanese Published Unexamined Patent Application No. 2003-59490    Patent Document 5: Japanese Published Unexamined Patent Application No. 2005-44785    Patent Document 6: Japanese Published Unexamined Patent Application No. 2006-253119    Patent Document 7: Japanese Patent No.4003759    Patent Document 8: Japanese Published Unexamined Patent Application No. 2002-145623    Patent Document 9: Japanese Published Unexamined Patent Application No. 2003-068298